I have 2 Maxtor Onetouch's, a 100GB and a 200GB. I'm pretty happy with them, but if you wanna use the backup software, I suggest you do it before you put anything on the drive...
Pretty much any external HDD will be the same. Since you claim to know next to nothing about PC's, I'd suggest researching the backup software that comes w/ the HDD, then use that as a deciding factor. Other things to look for would be the warranty and size over price (aka gigabyte per dollar).
Also remember that you want USB 2.0 as the interface. Stay away from USB 1.1 - unless you like waiting lengthy periods of time.
So with that combo, you can get a fairly rugged portable 120GB USB HD for about 100 bucks. That combo uses a standard desktop 3.5" HD so it's not a tiny portable. If you want something smaller, you could go with a laptop-sized (2.5") HD and an appropriate enclosure. For example:
So you can see that the preassembled route will cost you around $30 extra. But mounting a HD in a case and plugging cables is a trivial build. I've used both kinds with no problems. The 2.5" externals are great to have for an ultraportable storage device.
I agree. Size may not matter to woman but it does for hard drives, get as large as you can afford. They are always on sale at newegg. I just put together an external drive. A Peguien box and WD hard drive. Plugged the drive in, screwed everything together, plugged it in, and it worked. Only had problems with the set of USB ports on the front of one case, Worked on the rear and front and rear of a 2nd computer. Make sure it has a USB connection and if you don't have firewire, can save a little if you just get one with the USB connector not both.